This invention relates to the field of printing. In particular, this invention is drawn to whitegap reduction resulting from misregistration of color planes during color printing.
Color or polychromatic printing processes typically treat color images as a composition of color planes. The number of planes is a function of the selected color space. A red-green-blue (RGB) color space requires three planes. A cyan-magenta-yellow-black (CMYK) color space requires four planes.
Correct alignment of the individual planes is necessary for faithful reproduction of the source image. Spatial misalignment of the color planes during the printing process is referred to as misregistration. In CMYK color space, for example, misregistration can result in a whitegap artifact when one of the primary color planes (CMY) is printed next to black.
Various techniques for reducing whitegap artifacts have been developed. The color data may be modified after rasterization to reduce whitegap artifacts. One approach modifies luminance using a weighted combination of CMYK data. Typically, this approach requires significant computational resources due to the requirement of high-precision arithmetic calculations. This approach is also memory intensive because of the use of memory-resident templates needed to locate the edges.
Another approach uses a 72xc3x9772 pixel mask to implement a single pixel decision. This large mask size requires substantial memory to buffer 72 pixel rows. Another approach examines post-halftone data at one bit per pixel to perform the trapping and thus needs multiple passes to do the processing. Yet another approach performs a single bit pixel misregistration artifact correction. This approach, however, tends to fail when the registration errors are greater than one pixel.
In view of limitations of known systems and methods, methods of trapping to prevent a polychromatic printing artifact are described. One method includes the step of selecting a region of a source image comprising a plurality of concentric rings of pixels centered about a selected pixel. Each pixel has a plurality of toner values. Each of the toner values considered for trapping is associated with a non-black color plane. A trapped value is generated for each color plane of the selected pixel. The trapped value for a selected color plane corresponds to the maximum toner value for that color plane within the smallest ring having at least one non-black pixel. A trapped flag for each color plane of the selected pixel is generated. The trapped flag for a selected color plane is enabled only if the selected pixel is black and at least one ring not comprising the selected pixel includes a pixel that is non-black. The trapped value is substituted for the corresponding toner value of a given color plane of the selected pixel, if the trapped flag for the given color plane is enabled.
In one embodiment, an apparatus includes a pixel classifier classifying each pixel within a selected region of a source image as black or not black. The selected region is composed of a plurality of rings of pixels centered about a selected pixel. A plurality of ring trap logic circuits are associated with a selected color plane. Each trap logic circuit generates a ring trap flag and a ring toner value for an associated selected ring of pixels. The ring toner value is the maximum toner value for the selected color plane within the selected ring. The ring trap flag indicates whether at least one pixel within the selected ring is not black. A priority assignment unit coupled to the ring trap logic circuits provides a trapped flag if the ring trap flag for any ring other than the ring comprising the selected pixel is set. The priority assignment unit provides a trapped value corresponding to the toner value of the smallest ring having its ring trap flag set. A plurality of multiplexers are coupled to select one of the trapped value and the selected pixel""s original toner value for the corresponding color plane in accordance with a control signal. Logic circuitry generates the control signals in response to the trapped flag for each color plane and a trap enable signal. The multiplexer associated with a given color plane selects the corresponding trapped value only if the trapped flag for that color plane is set and the trap enable signal is enabled.
Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows below.